Ts-68 virus-transformed chick embryo fibroblasts (CEF) can uniquely express transformation by a shift in temperature of about 5C. Ts-68-infected and transformed quail cells showed about two times the thymidine kinase (TK) activity of normal quail cells. Ts-68-transformed CEF incubated at 41C showed about two times the TK activity of normal CEF incubated at the same temperature. The ts line incubated at 37C exhibited a higher protein concentration present in the post-microsomal, supernatant fraction than did the cells incubated at 41C. The TK from the ts-68-transformed CEF showed a higher temperature for optimal activity than did normal CEF. The effects of succinylacetone (SA), an inhibitor of the heme biosynthetic pathway, on growth and respiration of L1210 cells in vitro were examined. The cell growth with SA was normal for 2 days, after which time growth ceased. SA treatment of L1210 cells, even when growth ceased, caused less respiration inhibition than inhibition of growth after incubating the cells at high density. Both untreated and SA-treated cells responded similarly to mitochondrial inhibitors and uncoupling agents, suggesting that SA has no specific effect on respiration. Growth inhibition due to SA was not caused by heme depletion or impairment of respiration. Thus far, the only effect of SA which has been observed is the inhibition of the heme pathway.